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Helplog #37 -- Using Multiple Bias Sections

  • Monday, November 14 2005 @ 02:53 PM GMT
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Help Logs/Tips Question: I have a question concerning the colbias command in the noao.bias package of IRAF. I am working with a telescope which uses columns 1-48 and 2096-2158 as overscan regions. The bias is found by the colbias command. I was wondering if it is poissible to set up the parameters in such a way that both sections (so 1-48 and 2096-2158) are used for this purpose. Could you help me with this question?
NUMBER: 37
KEYWORDS: colbias, ccdproc, fit1d, prescan, overscan
DATE: Mon Nov 14 14:13:04 MST 2005
FROM: valdes

Q: I have a question concerning the colbias command in the noao.bias
package of IRAF. I am working with a telescope which uses columns
1-48 and 2096-2158 as overscan regions. The bias is found by the
colbias command. I was wondering if it is poissible to set up the
parameters in such a way that both sections (so 1-48 and 2096-2158)
are used for this purpose. Could you help me with this question?

A: It is not common to use both the prescan and overscan columns at
the same time. The standard IRAF tasks for handling bias (colbias
and ccdproc) do not allow use of more than one contiguous region
for bias. However, if you wish to do so you can use FIT1D to do
what you want. The parameters I recommend are:


I R A F
Image Reduction and Analysis Facility
PACKAGE = imfit
TASK = fit1d

input = Images to be fit
output = Output images
(axis = 1) Axis to be fit
type = difference Type of output (fit, difference, ratio)
(interac= no) Set fitting parameters interactively?
(sample = 1-48,2096-2158) Sample points to use in fit
(naverag= -100) Number of points in sample averaging
(functio= chebyshev) Fitting function
(order = 2) Order of fitting function
(low_rej= 0.) Low rejection in sigma of fit
(high_re= 0.) High rejection in sigma of fit
(niterat= 1) Number of rejection iterations
(grow = 0.) Rejection growing radius in pixels
(graphic= stdgraph) Graphics output device
(cursor = ) Graphics cursor input
(mode = ql)

The type of "difference" subtracts the bias, the "sample"
parameter defines the data to use for the bias. The value of
-100 says to take the median of each bias region (a median is
good to avoid bad pixels and cosmic rays), and the function and
order fit a line between the two regions. The value of the fit
is subtracted from each pixel in the image. Read the help page
for this task for the various options.

To later trim away the bias regions you would use imcopy with image
sections.

cl> imcopy <image>[49:2095,*] <newimage>
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